Process for producing polyolefin films

Abstract

Process for producing an oriented polyolefin film using a multi-component polymer composition provided by a primary polymer component comprising an isotactic propylene homopolymer produced by the polymerization of propylene in the presence of a Ziegler-Natta catalyst and a secondary polymer component comprising a metallocene polymerized isotactic propylene polymer having a xylene solubles content which is less than that of the primary polymer component. The secondary polymer component is a propylene homopolymer or an ethylene-propylene random copolymer having an ethylene content of about 0.5 wt. % or less. The polymer components are separately co-extruded to form a multilayer co-extrudate in which the primary polymer component is present in an amount greater than the secondary polymer component. The multilayer co-extrudate is oriented in at least one direction to form an oriented film.

1 . In a process for producing a polyolefin film comprising: 
 (a) providing a primary polymer component comprising an isotactic propylene homopolymer produced by the polymerization of propylene in the presence of a Ziegler-Natta catalyst;    (b) providing a secondary polymer component comprising a metallocene-polymerized propylene polymer having a xylene solubles content which is less than the xylene solubles content of said primary polymer component selected from the group consisting of: 
 (i) a propylene homopolymer; and  
 (ii) an ethylene-propylene random copolymer having an ethylene content of about 0.5 wt. % or less;  
   (c) separately co-extruding said primary and secondary polymer components together to form a multilayer co-extrudate of said primary and secondary polymer components in which the primary polymer component is present in an amount which is greater than the amount of said secondary polymer component; and    (d) forming an oriented film of said polymer components by stretching said multilayer co-extrudate in at least one direction.    
   
   
       2 . The process of  claim 1  in which said primary polymer component is present in said co-extrudate in an amount within the range of 80-90 wt. % and said secondary polymer component is present in an amount within the range of 10-20 wt. %.  
   
   
       3 . The process of  claim 2  in which the primary polymer component has a xylene solubles content which is greater than the xylene solubles content of said secondary polymer component by a factor of at least 3.  
   
   
       4 . The process of  claim 1  wherein said secondary polymer component has a melting temperature which is lower than the melting temperature of said primary polymer component.  
   
   
       5 . The process of  claim 1  wherein said multilayer co-extrudate comprises a base layer formed of said primary polymer component and film layer on the opposed surfaces of the base layer formed of said secondary polymer component.  
   
   
       6 . The process of  claim 1  wherein said film is biaxially oriented by stretching said multilayer co-extrudate in longitudinal and transverse directions.  
   
   
       7 . The process of  claim 6  wherein said multilayer co-extrudate exhibits a transverse direction yield stress at a designated preheating time which is less than the transverse direction yield stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       8 . The process of  claim 6  wherein said multilayer co-extrudate exhibits a machine direction final draw stress at a designated preheating time which is less than the machine direction final draw stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       9 . The process of  claim 6  wherein said multilayer co-extrudate exhibits a transverse direction final draw stress at a designated preheating time which is less than the transverse direction final draw stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       10 . The process of  claim 6  wherein said multilayer co-extrudate exhibits a transverse direction apparent toughness at a designated preheating time which is less than the transverse direction apparent toughness at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       11 . The process of  claim 1  wherein said secondary polymer component is a metallocene polymerized propylene homopolymer having a xylene solubles content of less than 0.5 wt. %.  
   
   
       12 . The process of  claim 11  wherein said metallocene polymerized propylene homopolymer has a melt flow rate of less than 5 decigrams/minute.  
   
   
       13 . The process of  claim 12  wherein said primary polymer component is a high crystallinity isotactic propylene homopolymer having a xylene solubles content of no more than 1 wt. %.  
   
   
       14 . The process of  claim 12  wherein said primary polymer component is a isotactic propylene homopolymer having a xylene solubles content of at least 3 wt. % and a melt flow index which is less than the melt flow index of said secondary polymer component.  
   
   
       15 . In a process for producing a polyolefin film comprising: 
 (a) providing a primary polymer component comprising an isotactic propylene homopolymer produced by the polymerization of propylene in the presence of a Ziegler-Natta catalyst, said isotactic propylene homopolymer having a xylene solubles content of at least 3 wt. %;    (b) providing a secondary polymer component comprising a metallocene-polymerized propylene polymer having a xylene solubles content which is less than the xylene solubles content of said primary polymer component selected from the group consisting of: 
 (i) a propylene homopolymer; and  
 (ii) an ethylene-propylene random copolymer having an ethylene content of about 0.5 wt. % or less;  
   (c) extruding said primary and secondary polymer components together to provide an extrudate of said primary and secondary polymer components in which the primary polymer component is present in an amount which is greater than the amount of said secondary polymer component; and    (d) stretching said extrudate in longitudinal and transverse directions to form a biaxially oriented film of said polymer components which exhibits a transverse direction yield stress at a designated preheating time which is less than the transverse direction yield stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.    
   
   
       16 . The process of  claim 15  wherein said biaxially oriented film exhibits a machine direction final draw stress at a designated preheating time which is less than the machine direction final draw stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       17 . The process of  claim 15  wherein said biaxially oriented film exhibits a transverse direction final draw stress at a designated preheating time which is less than the transverse direction final draw stress at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       18 . The process of  claim 15  wherein said biaxially oriented film exhibits a transverse direction apparent toughness at a designated preheating time which is less than the transverse direction apparent toughness at said designated preheating time of a corresponding film formed by extruding said primary polymer component alone.  
   
   
       19 . The process of  claim 15  in which said primary polymer component is present in said extrudate in an amount within the range of 80-90 wt % and said secondary polymer component is present in an amount within the range of 10-20 wt. %.  
   
   
       20 . The process of  claim 19  in which the primary polymer component has a xylenes content which is greater than the xylenes content of said secondary polymer component by a factor of at least 3.  
   
   
       21 . The process of  claim 19  wherein said secondary polymer component has a melting temperature which is lower than the melting temperature of said primary polymer component.